Fix missing nbformat version

[fcollonval]

Fix an inconsistency to allow missing format information at

jupyter_ydoc/jupyter_ydoc/ynotebook.py

Line 205 in 2b8e788

if "id" in cell and meta["nbformat"] == 4 and meta["nbformat_minor"] <= 4:

This is to aligned with the tolerance implemented a bit later:

jupyter_ydoc/jupyter_ydoc/ynotebook.py

Lines 216 to 221 in 2b8e788

	return dict(
	cells=cells,
	metadata=meta.get("metadata", {}),
	nbformat=int(meta.get("nbformat", 0)),
	nbformat_minor=int(meta.get("nbformat_minor", 0)),
	)

[davidbrochart]

Thanks @fcollonval.

[davidbrochart]

In the future, could you make a PR from your fork of jupyter_ydoc?